1. Field of the Invention
The present invention relates to a photoelectric conversion device including a single crystal semiconductor or a polycrystalline semiconductor and to a manufacturing method of the photoelectric conversion device.
2. Description of the Related Art
The industrial growth has been boosting energy consumption worldwide. The mainly used energy such as oil, coal, and natural gas produces a large amount of carbon dioxide, which has been a factor of drastic global warming in recent years. In response to that, photovoltaic power generation which produces less amount of carbon oxide and is eco-friendly is getting popular.
Some photovoltaic power generation utilize solar heat, while many others employ photoelectric conversion devices (also called solar cells or photoelectromotive devices) by which light energy is converted into electric energy with use of the photoelectric characteristics of semiconductor.
Photoelectric conversion devices are already available in the market and the production thereof has been expanding year by year with government support for solar cells around the world. For example, the production of solar cells around the world in 2006 is 2521 MW, which has increased by over 40% per annum. Above all, photoelectric conversion devices including a crystalline semiconductor have become popular worldwide, and a large part of the production is occupied by the devices including single crystal silicon substrates or polycrystalline silicon substrates.
As for a crystalline photoelectric conversion device using silicon as a material, a thickness of about 10 μm is enough to absorb sunlight. However, a single crystal silicon wafer used in the product has a diameter of 6 inches (150 mm) to 12 inches (300 mm) and a thickness of 600 μm to 800 μm, whereas a polycrystalline silicon wafer used in the product has a length of 100 mm to 150 mm on each side and a thickness of 200 μm to 350 μm. That is to say, the thickness of a single crystal silicon substrate or a polycrystalline silicon substrate is several tens times or more the necessary thickness of a photoelectric conversion device and the substrate, which is a material, is not used effectively at present. In an extreme case, when a single crystal silicon substrate or a polycrystalline silicon substrate is used in a photoelectric conversion device, most part of the substrate only functions as a structural body that keeps the shape of the photoelectric conversion device.
With the increase in production of photoelectric conversion devices year by year, short of supply and rise of cost of silicon, which is the material of single crystal silicon or polycrystalline silicon, have become significant problems in the industry. As for the supply-demand balance of polycrystalline silicon, which had been excess in supply reflecting semiconductor recession, polycrystalline silicon is now short of supply since around fiscal 2005 because of drastic expansion of the solar cell market in addition to the recovery of the semiconductor (LSI) industry. Major silicon suppliers in the world have already tried to increase capability of silicon production, though the increase in demand outweighs the capability and the short of supply seems to continue for some time.
A variety of structures of photoelectric conversion devices have been disclosed. In addition to a typical structure in which an n type or a p type diffusion layer is formed in a single crystal silicon substrate or a polycrystalline silicon substrate, a stacked type photoelectric conversion device in which different kinds of unit cells are combined such that a unit cell including a single crystal semiconductor or a polycrystalline semiconductor and a unit cell including an amorphous semiconductor or a microcrystalline semiconductor are combined is known (e.g., see Patent Document 1: Examined Patent Application Publication No. H6-044638). However, the photoelectric conversion device as aforementioned also needs to use a single crystal semiconductor substrate or a polycrystalline semiconductor substrate.
In recent years, development has been actively conducted on an SOI (silicon on insulator) structure which includes a single crystal silicon thin film formed over a substrate having an insulating surface. Although an SOI substrate is expensive, the cost can be decreased as compared with a bulk single crystal silicon substrate as long as an inexpensive substrate such as a glass substrate can be used as a supporting substrate. Moreover, the consumption of silicon, which is the material, can be decreased. For example, a method of manufacturing an SOI substrate in which a single crystal silicon layer is fixed to a glass substrate by a hydrogen ion implantation separation method is known (e.g., see Patent Document 2: Japanese Published Patent Application No. H11-097379).